Power factor correction stages are often used as part of AC-DC converters. AC-DC converters are used to transform power from a supply voltage into a voltage required by an application. Typically the supply voltage is a mains voltage, and in such cases, in particular, requirements may be imposed such as the maximum allowable mains harmonics or minimum allowable power factor. In such circumstances it is common to include in the AC-DC converter a power factor correction stage. In order to minimise the power losses in the converter, it is desirable to maximise the efficiency of the power factor correction stage.
A commonly used topology for power factor correction stages is the boost converter, an example of which is shown in FIG. 1. The converter 1 comprises a coil (L1), in series with the input voltage uin. The other end of the coil is connected to a node to between two switches SW1 and SW2. The first switch SW1 is connected to ground; the second switch (SW2) is typically implemented as a passive switch or diode as shown in FIG. 1, and is connected between node 2 and the output. An output capacitor (C1) is connected between the output and ground. For such a boost converter, the output voltage, uout, is larger than the input voltage uin. The coil L1, together with switch SW1 and diode SW2 may be termed a “power cell”.
In operation, the output current of source uin can be controlled by controlling the on-time (Ton) of switch SW1. A switching cycle starts by turning on switch SW1 for a time Ton. In this phase the total input voltage uin is across the coil. Because the input voltage is positive the current iL(t) through coil L1 will rise:
                                          i            L                    ⁡                      (            t            )                          =                              t            ⁢                                          u                in                                            L                1                                              +                                                    i                L                            ⁡                              (                0                )                                      .                                              (        1        )            
where iL(0) is the current at time t=0.
After a time Ton SW1 is turned off. As the current iL(Ton) through L1 wants to flow continuously the current is automatically commutated to the diode. The output voltage uout is always higher than the input voltage uin. Therefore, the current through L1 will decrease:
                                          i            L                    ⁡                      (            t            )                          =                                            i              L                        ⁡                          (                              T                on                            )                                +                      t            ⁢                                                            u                  in                                -                                  u                  out                                                            L                1                                                                        (        2        )            
Switch SW2 is turned off after a further time Toff, when the current iL becomes zero or when SW1 is turned on again. In the case that iL(Toff) is zero it is said that the converter works in discontinuous conduction mode (DCM), as shown by the bottom curve 21 of FIG. 2. In the case that it is allowed that iL(Toff) is not always zero at the moment SW1 is turned on it is said that the converter is working in continuous conduction mode (CCM), as shown by the top curve 22 of FIG. 2. A special case is distinguished if at the time IL(Toff) becomes zero immediately SW1 is turned on again. In this case the converter works in boundary condition mode (BCM), shown by the middle curve 23 of FIG. 2.
Typically, SW1 is implemented as a MOSFET; a diode is an effective passive switch SW2, since a diode will naturally turn on if the voltage drop is positive and naturally turn off as soon as the current through it becomes zero.
In DCM or BCM, the current through the coil L1 can show large peaks, resulting in large ripples in any input filter and the output capacitor C1. In order to reduce this ripple, it is known to use two converters of the same type operating at 180° out of phase, or even three converters with respective 120° phase shifts, etc. Such a converter may be termed an “interleaved converter”.
FIG. 3 shows such a converter. The figure shows a first power cell having a coil or inductor L1 connected between input voltage Uin and the node between switch first switch S1 and first diode D1, together with a second power cell having inductor L2 connected between the input voltage and the node between second switch S3 and second diode D2.
FIG. 4 shows the current i1 through the coil L1, (top curve 41), the current i2 through coil L2 (middle curve 42), and the combined current i1+i2 (bottom curve 43) for such a power factor converter, where both power cells are operating in discontinuous mode and are 180° out of phase.
Control in CCM is relatively straightforward since the frequency is fixed, but is more complicated in BCM in order to maintain the required 180° phase shift. Furthermore, in this interleaved solution both converters are always working, and thus both contribute to power losses.
Publications JIUN-REN TSAI ET AL: “Interleaving Phase Shifters for Critical-Mode Boost PFC” IEEE TRANSACTIONS ON POWER ELECTRONICS, IEEE SERVICE CENTER, PISCATAWAY, NJ, UK, vol. 23, no. 3, 1 May 2008, pages 1348-1357 and HUBER L ET AL: “Closed-Loop Control Methods for Interleaved DCM/CCM Boundary Boost PFC Converters” APPLIED POWER ELECTRONICS CONFERENCE AND EXPOSITION, 2009. APEC 2009. TWENTY-FOURTH ANNUAL IEEE, IEEE, PISCATAWAY, NJ, USA, 15 Feb. 2009, pages 991-997 disclose such interleaved solutions as does United States Patent Application, Publication US2008/0316783.
It is desirable to provide an alternative control arrangement with improved efficiency relative to a single power cell converter or to such an interleaved converter.